The invention relates to improvements in rolls of the type often used in rotary printing presses wherein they cooperate with printing cylinders to define nips for running webs of paper, foil or other strip or sheet material, for example, webs or strips of wrapping or packing material.
It is customary to employ in a rotary printing press a deformable roll which cooperates with the printing cylinder and whose deformability is intended to be sufficiently pronounced to ensure that the nip of such roll with the printing cylinder will have a desired width all the way from the one to the other end of the printing cylinder. In other words, the roll should cooperate with the printing cylinder in order to ensure that the printing cylinder will bear against the running web with equal force in each and every region of the nip. This guarantees that the image which is applied to the running web is sharp along the marginal portions as well as in each region between the marginal portions of the web.
Heretofore known rolls are capable of meeting the above outlined requirements if the nips of such rolls with printing cylinders are relatively short. However, conventional rolls are unsatisfactory when the nip is very lone (e.g., up to and even in excess of three meters) and/or when a relatively long printing cylinder is driven at a high rotational speed. It has been found that the dynamic stability of relatively long conventional rolls is unsatisfactory as well as that the image which is applied to a web running through a nip having a length of several meters and being defined in part by a rapidly rotating cylinder is non-uniform because the outline of the roll does not adequately conform to the outline of the printing cylinder. The latter undergoes deformation when the printing press is in use and, once its length and/or its RPM exceed a certain value, a conventional roll is incapable of conforming to the outline of the deformed printing cylinder when the press is in actual use. In order to adequately conform to the shape of a deformed printing cylinder and to the shape of a printing cylinder whose deformation varies during each revolution, the roll would have to yield to often extensive radial deforming and/or twisting stresses at a frequency and/or to an extent which cannot be achieved with heretofore known rolls. This is due, in part, to the fact that the mass of a conventional roll which cooperates with a long or very long printing cylinder (e.g., a printing cylinder which is 3 meters long) is excessive so that the inertia of the roll prevents it from rapidly and repeatedly conforming to the changing shape of the printing cylinder. Such relatively heavy rolls tend to vibrate, and this also affects the quality of the image which is applied to a web running through the nip of the roll and a printing cylinder. In fact, the frequency and/or amplitude of vibrations which develop in printing presses employing long and heavy printing cylinders and long and heavy companion rolls can be so pronounced that the presses must be operated at much less than their rated speed, with attendant losses in output, in order to avoid damage to or destruction of the machine.